I don't know what your metric for reliability is... but I would say that the messaging / serialization format can hardly be blamed for reliability problems s.a. service uptime or service responsiveness etc. Such problems usually arise at a level where details like the choice of messaging format are not important.

Messaging format may affect application performance though. It would affect metrics s.a. throughput or bandwidth.

In a way that is more difficult to measure, a messaging format can affect the number of bugs created by developer using it and indirectly the delivery times. But this can be mitigated by tooling (i.e. Protobuf isn't self-documenting, so if you don't have the schema files, you can't interpret the messages, but you can write a tool that you can feed the schema definitions and then use the tool to interpret the messages).

> It's one of the few things Google _hasn't_ deprecated

You might be unaware of it, but there were Protobuf v1, v2, and now we are at v3. Even though it's not documented, v3 supports most of v2, but not all of it (I think v1 was never used outside of Google itself). Google never properly released Protobuf, so, they can't really deprecate it. Even their formal grammar is full of errors.